The present invention relates to a method of determining depth values for the bottom profile of a body of water from pulse transit times and sounding directions by means of a fan beam echo sounder that measures transversely to the longitudinal axis of the ship, whose transducer array is installed in each case at an angle obliquely to the vertical elevation axis of the ship and wherein the pulse transit times are measured and stored in association with the sounding directions, with the sounding directions .alpha..sub.oi being calculated from data furnished by the transducer array and by the transducer signals as well as by means of a predeterminable sound velocity c.sub.o at the surface of the transducer array.
To ensure the safety of shipping lanes, it is necessary to precisely monitor and record the depth of bodies of water that are dependent on flow. For this purposes, maps of the bottom profile of the body of water are generally produced by means of echo soundings. In the past, it has been the custom to employ individual beam echo sounders with which the bottom profile of the body of water was covered dot by dot by means of vertical soundings, with several transducers attached to lateral outrigger arms recording and processing echo signals in parallel operation. More recently, there has been an increased use of fan beam echo sounders in which a transducer array simultaneously processes a plurality of sounding directions. Due to the absence of outrigger arms, vessels equipped with fan beam echo sounders are able to travel very quickly on their measuring trips in which case. However, the sounding directions are no longer vertical but extend obliquely to the sides transversely to the longitudinal axis of the ship. Since high demands are placed on accuracy if more than eighty sounding directions are involved and water depths to more than 10,000 m, the sounding directions must be set very precisely.
European Patent 0,140,258, which corresponds to U.S. Pat. No. 4,611,313, discloses the use of a fan beam echo sounder for the determination of bottom profiles. For a determination of depth values, the transmitted sound beams in that arrangement are directed out to the sides transversely to the longitudinal direction of the ship at a large beam angle and in the direction of travel at a very narrow beam angle so that a narrow strip of the bottom profile is covered with sound. By electronic processing in a receiving device, a fan of narrow received beams is then oriented onto the bottom profile strip being surveyed so as to pick up the reflected and back-scattered pulse signals. From the set sounding direction of the received beams and the pulse transit time, the depth value of a quasi dot-shaped element in the strip of the water bottom profile can then be determined. The precise position of this element and thus the precise determination and association of the depth value is decisively dependent on the predetermined sounding direction.
Although European Patent 0,140,258 (U.S. Pat. No. 4,611,313) discloses the surveying of the bottom profile of a body of water by means of a fan beam echo sounder and to determine depth values from the transit times of the received echoes on the basis of the oblique sounding directions, the curved path of the sound beam caused by changes in the sound velocity as a result of the depth of the water and its salt content lead to considerable measuring inaccuracies in the determination of the depth values. These measuring inaccuracies are compensated by pivoting the sounding fan and making a calibration measurement to compensate for the distortions in the measuring results due to the curvatures in the sound beam. However, the prior art method does not consider a measuring error caused by the direction formation of the transducer array.
For a determination of the sounding directions of the transducer array from the individual fan sectors, an assumed sound velocity directly at the surface of the transducers is considered. If it is in error, the sounding directions and the depth values determined therefrom will be faulty. According to the prior art the sound velocity is measured, for example, by bathythermography. Such a method is complicated and presumes that the vessel is not traveling. Sound velocity measuring probes attached underneath the vessel are also unsuitable since they furnish inconsistent and often falsified measuring results due to the flow conditions at the hull of the vessel.